The wealth of the United States has been created largely through the exploitation of cheap energy provided by the past abundance of fossil fuels. Because of the increasing shortages of natural gas in North America, the continued reliance on oil suppliers located volatile regions, the approaching worldwide shortages of oil, and because of the growing danger of global warming that may be caused by the combustion of fossil fuels, clean reliable sources of renewable energy are needed.
Many of the efforts to develop power generation systems fueled by renewable energy sources have been focused on wind energy. Although wind powered generating systems provide many benefits, they have a significant drawback. Specifically, wind direction and speed are in a constant state of flux. Wind speeds can fluctuate hourly and have marked seasonal and diurnal patterns. They also frequently produce the most power when the demand for that power is at its lowest. This is known in the electricity trade as a low capacity factor. Low capacity factors, and still lower dependable on-peak capacity factors, are notable shortcomings of wind power generation.
In contrast to the winds, some deep ocean currents are driven largely by relatively steady Coriolis forces. The fact that such ocean currents are not subject to significant changes in direction or velocity makes sub-sea power generation somewhat more desirable than the intermittent power produced by wind-driven turbines. The book, Ocean Passages of the World (published by the Hydrographic Department of the British Admiralty, 1950), lists 14 currents that exceed 3 knots (3.45 mph), a few of which are in the open ocean. The Gulf Stream and the Kuro Shio are the only two currents the book lists having velocities above 3 knots that flow throughout the year. Both of these currents are driven by the Coriolis force that is caused by the Earth's eastward rotation acting upon ocean currents produced by surface trade winds. Because these currents are caused largely by the Earth's rotation, they should remain constant for a substantial period barring significant changes in local geography.
The Gulf Stream starts roughly in the area where the Gulf of Mexico narrows to form a channel between Cuba and the Florida Keys. From there the current flows to the northeast through the Straits of Florida, between the mainland of the United States and the Bahamas, flowing at a substantial speed for some 400 miles. The peak velocity of the Gulf Stream is achieved off of the coast of Miami, Fla., where the Gulf Stream is about 45 miles wide and 1,500 feet deep. There, the current reaches speeds of as much as 6.9 miles per hour at a location between Key Largo, Fla. and North Palm Beach, Fla., and less than 18 miles from shore. Farther along it is joined by the Antilles Current, coming up from the southeast, and the merging flow, broader and moving more slowly, continues northward and then northeastwardly, as it roughly parallels the 100-fathom curve as far as Cape Hatteras, N.C.
The Kuro Shio is the Pacific Ocean's equivalent to the Gulf Stream. A large part of the water of the North Equatorial current turns northeastward east of Luzon and passes the east coast of Taiwan to form this current. South of Japan, the Kuro Shio flows in a northeasterly direction, parallel to the Japanese islands, of Kyushu, Shikoku, and Honshu. According to Ocean Passages of the World, the top speed of the Kuro Shio is about the same as that of the Gulf Stream. The Gulf Stream's top flow rate is 156.5 statute miles per day (6.52 mph) and the Kuro Shio's is 153 statute miles per day (6.375 mph).
Other possible sites for the underwater generators are the East Australian Coast current, which flows at a top rate of 110.47 statute miles per day (4.6 mph), and the Agulhas current off the southern tip of South Africa, which flows at a top rate of 139.2 statute miles per day (5.8 mph). Another possible site for these generators is the Strait of Messina, the narrow opening that separates the island of Sicily from Italy, where the current's steady counter-clockwise rotation is produced primarily by changing water densities resulting from evaporation in the Mediterranean. Oceanographic current data may suggest other potential sites.
Submersible turbine generating systems can be designed to efficiently produce power from currents flowing as slowly as 3 mph—if that flow rate is consistent—by increasing the size of the turbines in relation to the size of the generators, and by adding more gearing to increase the shaft speeds to the generators. Because the Coriolis currents can be very steady, capacity factors of between 70 percent and 95 percent may be achievable. This compares to historical capacity factors for well-located wind machines of between 23 percent and 30 percent. Because a well-placed submersible water turbine will operate in a current having even flow rates, it may possible for it to produce usable current practically one hundred percent of the time.
Moreover, increasing human ingress into the oceans makes undersea power generation desirable. Historically, submarines have had to periodically surface and dock at shore based ports for maintenance that has included recharging or replacing electric batteries and/or receiving temporary electric power during the maintenance of their on-board generators. Such needs to periodically travel to shore based facilities have undesirably limited the mission capabilities of many submarines. A suitable deep sea power generation facility could provide opportunities for submarines to obtain electric power for maintenance while remaining submerged and without diversion from the open ocean to a shore location. Additionally, as the number of underwater scientific observatories increases, so does the need to generate power for the observatories at the observatory sites. Further, whether engaged in military, scientific, commercial, or recreational activities humans need potable water. Potable water can be produced from sea water, but such production facilities typically require electricity.
Although the needs are numerous, viable sub-sea power generation has presented notable challenges. For example, rotating electric generators produce heat. The electric current flowing through the conductors, both in the stator and rotor, produces heat because of the electrical resistance. In addition, heat is generated in the steel of the rotor armature core by the changing magnetic fluxes and bearing, shaft, and gear friction produces heat as well. Although the heat loss in large generators is typically only on the order of about 1 percent of output, this is still considerable. For example, a pair of generators producing 1,200 kW might have a loss of 12 kW, which is equivalent to 40,973 BTU per hour. Therefore, a liquid cooling system is desirable for dissipation of heat produced by a sub-sea power generation system. Additionally, maintaining proper horizontal, vertical, and azimuthal turbine positioning relative to ocean current depths and directions for optimizing capacity factors in operation of sub-sea power generation systems has been challenging. Another challenge has been that deeply submerging power generation units has made them less readily accessible for servicing and repair.